Pursuant to 37 C.F.R. 1.821(c), a sequence listing is submitted herewith as an ASCII compliant text file named “GENUP0035US_ST25.txt”, created on Sep. 9, 2015 and having a size of ˜16 kilobytes. The content of the aforementioned file is hereby incorporated by reference in its entirety.
1. Field of the Invention
This invention relates to regulation of cell growth, and more particularly to regulation of cancer cell growth. In particular, MUC1 peptides derived from a particular region of the MUC1 cytoplasmic domain have been shown to inhibit MUC oligomerization and nuclear translocation, causing inhibition and even death of MUC1-expressing tumor cells.
2. Related Art
The HER2/ERBB2 receptor tyrosine kinase (RTK) is overexpressed in approximately 20% of human breast cancers and is associated with aggressive disease and poor survival (Slamon et al., 1987 and Hynes et al., 2005). HER2 forms heterodimers with HER3 and thereby activates the PI3K->AKT pathway (Holbro et al., 2003). Downregulation of HER3 in HER2-dependent breast cancer cells is thus associated with inhibition of PI3K signaling and proliferation (Holbro et al., 2003). Trastuzumab is a humanized monoclonal antibody that binds to the HER2 extracellular domain and destabilizes ligand-independent HER2/HER3 complexes (Junttila et al., 2009). Targeting of HER2 with trastuzumab in HER2-overexpressing breast cancer cells also suppresses constitutive activation of the PI3K->AKT pathway (Junttila et al., 2009), decreases HER2 levels (Klapper et al., 2000 and Scaltriti et al., 2009), and induces GI1 arrest by stabilizing the CDK inhibitor p27 (Shin et al., 2002). Trastuzumab extends the overall survival of certain patients with HER2-overexpressing breast cancers when used as monotherapy or in combination with chemotherapy (Slamon et al., 2001, Romond et al., 2005 and Spector and Blackwell, 2009). However, many patients exhibit de novo unresponsiveness to trastuzumab or develop acquired resistance after treatment (Spector and Blackwell, 2009). Trastuzumab resistance has been associated with constitutive activation of the PI3K pathway as a result of phosphatase and tensin homolog (PTEN) deficiency (Nagata et al., 2004) or PIK3CA gene mutations (Berns et al., 2007). PTEN has also been linked to SRC activation and thereby trastuzumab resistance in breast cancer cells and in breast tumors (Zhang et al., 2011). Additional mechanisms of resistance have included expression of a truncated p95HER2 that lacks the trastuzumab binding domain (Scaltriti et al., 2007), heterodimerization with other RTKs (Nahta et al. 2005, Shattuck et al., 2008 and Huang et al., 2010) and downregulation of HER2 expression (Mittendorf et al., 2009). Other studies have shown that resistance of HER2-overexpressing breast cancer cells to trastuzumab is conferred by (i) upregulation of cyclin E and an increase in CDK2 activity (Scaltriti et al., 2011), and (ii) decreased expression of the PPM1H phosphatase that regulates p27 stability (Nahta et al., 2004 and Lee-Hoeflich et al., 2011). These findings have provided the experimental basis for designing trials that target pathways associated with trastuzumab resistance to reverse unresponsiveness to this agent in the clinic.
MUC1 is a heterodimeric protein that constitutively associates with HER2 on the surface of breast cancer cells (Li et al., 2003 and Kufe, 2013). MUC is translated as a single polypeptide that undergoes autocleavage into N-terminal (MUC1-N) and C-terminal (MUC1-C) fragments, which in turn form a stable complex at the cell membrane (Kufe, 2009). The MUC1-N/MUC1-C heterodimer is positioned at the apical border of breast epithelial cells and is sequestered from RTKs that are expressed at the baso-lateral membranes (Kufe, 2013 and Kufe, 2009). However, with loss of apical-basal polarity as a result of stress or transformation. MUC1 is repositioned over the entire cell membrane and interacts with RTKs such as HER2 (Li et al., 2003, Kufe, 2013 and Kufe, 2009). MUC1-N, the mucin component of the heterodimer, is shed from the cell surface (Kufe, 2013 and Kufe, 2009). The MUC1-C subunit spans the cell membrane and includes a 58 amino acid (aa) extracellular domain, a 28 as transmembrane domain and a 72 as cytoplasmic domain. MUC1-C associates with RTKs through extracellular galectin-3 bridges (Duraisamy et al., 2007). In addition, the MUC1-C cytoplasmic domain functions as a substrate for phosphorylation by RTKs and SRC (Kufe, 2013 and Kufe, 2009). The MUC1-C cytoplasmic domain also contains a YHPM motif that, when phosphorylated on tyrosine, functions as a binding site for PI3K p85 SH2 domains (Raina et al., 2011). Overexpression of MUC1-C, as found in breast cancers, is associated with activation of the PI3K→AKT pathway (Raina et al., 2004). These findings and the demonstration that MUC1-C overexpression is sufficient to induce anchorage-independent growth and tumorigenicity (Li et al., 2003 and Huang et al., 2005) provided the basis for developing agents that block the MUC1-C transforming function (Kufe, 2009). In this respect, the MUC1-C cytoplasmic domain contains a CQC motif that is necessary for its dimerization and function (Leng et al., 2007). Accordingly, cell-penetrating peptides that bind to the MUC1-C CQC motif are effective in inhibiting growth and inducing death of human breast cancer cells growing in vitro and as xenografts in mice (Raina et al., 2009). MUC1-C inhibitors were also found to be effective in (i) blocking the interaction between the MUC1-C cytoplasmic domain and PI3K p85 in vitro, and (ii) suppressing constitutive activation of the PI3K→AKT pathway in cells (26). However, the effects of MUC1-C inhibition on (i) the interaction between MUC1-C and HER2, and (ii) HER2 signaling in breast cancer cells are not known.